In the related art, as an example of a compound semiconductor light-emitting element, a p-n junction light emitting diode (LED) is known. For example, a GaP-based LED or the like is known in the art, in which a GaP layer obtained by epitaxially growing a conductive gallium phosphide (GaP) single crystal on a substrate is used as a light-emitting layer. In addition, there are also known LEDs having red, orange-yellow, to green color ranges having a light-emitting layer including a mixed crystal of aluminum.gallium arsenide (having a composition of AlXGaYAs: 0≦X,Y≦1, and X+Y=1) or a mixed crystal of aluminum.gallium.indium phosphide (having a composition of AlXGaYInZP: 0≦X,Y,Z≦1, and X+Y+Z=1). In addition, there are known short-wavelength LEDs having a near-ultraviolet, blue, or green color range using a gallium nitride-based compound semiconductor layer such as gallium-indium nitride (having a composition of GaαInβN: 0≦α,β≦1, α+β=1) as the light-emitting layer.
For example, in the AlXGaYInZP-based LED described above, a conductive n-type or p-type light-emitting layer is formed on a conductive p-type or n-type gallium arsenide (GaAs) single crystal substrate. In addition, in the blue color LED, single crystals such as sapphire (α-Al2O3 single crystal) having an electric insulation are used in the substrate. In addition, in the short-wavelength LEDs, silicon carbide (SiC) having a cubical crystal structure (3C crystal type) or a hexagonal crystal structure (4H or 6H crystal type) is also used in the substrate. In addition, the light-emitting element is formed by providing, for example, a first conductive translucent electrode and a second conductive electrode in a predetermined position on the semiconductor wafer obtained by stacking the semiconductor layer on such a substrate.
Here, particularly, in the case of the gallium nitride-based compound semiconductor light-emitting element, a gallium nitride-based compound semiconductor is formed on a substrate containing various oxides or III-V group compounds as well as sapphire single crystals using a metal-organic chemical vapor deposition (MOCVD) method, a molecular beam epitaxial (MBE) method, or the like.
In the related art, as a characteristic of the gallium nitride-based compound semiconductor light-emitting element, an electric current diffusion to the horizontal direction is small, and the electric current is applied to only the semiconductor layer immediately underlying the electrode. Therefore, the light emitted from the light-emitting layer is blocked by the electrode, and it is difficult to extract the light to the outside. In this regard, such a gallium nitride-based compound semiconductor light-emitting element is typically configured such that a translucent electrode is used as the positive electrode, and the light is extracted from the positive electrode.
As the translucent electrode, for example, an Ni/Au stack structure or a conductive material known in the art such as ITO is used. In addition, recently, it has been proposed to use an oxide-based translucent electrode containing In2O3 or ZnO as a main component because they have a superior translucency (e.g., refer to Patent Citation 1). The ITO used in the light-emitting element disclosed in the Patent Citation 1 is most widely used as the translucent electrode. Such an ITO contains In2O3 where SnO2 of 5 to 20 mass % is doped and can be obtained as a conductive oxide film having a low resistivity less than or equal to 2×10−4 Ωcm.
In order to improve light extraction efficiency from the translucent electrode, it has been proposed to emboss the light extraction surface of the translucent electrode (e.g., refer to Patent Citation 2). In the translucent electrode provided in the light-emitting element disclosed in Patent Citation 2, micro-crystals are formed immediately after the ITO film having a low resistance is formed. Therefore, in order to emboss the ITO, it is necessary to use an etching solution such as an aqueous solution of ferric chloride (FeCl3) or a hydrochloric acid (HCL). However, since the etching rate is fast in a wet etching process using such a strong acidic etching solution, it is difficult to control, and burrs are easily generated in the edge portions of the ITO film. In addition, since the overetching is easily generated, a product yield may be degraded.
In order to address the problem in the embossing process of the translucent electrode as disclosed in Patent Citation 2, it has been proposed to relatively smoothly perform the etching process without a strong acidic solution such as the etching solution by forming an amorphous IZO film using a sputtering method (e.g., refer to Patent Citation 3). According to the method disclosed in Patent Citation 3, burrs are hardly generated, and etching is not excessively performed in comparison with the etching process using strong acid. In addition, it is possible to readily perform micro-machining for improving the light extraction efficiency.
However, since the amorphous IZO film is inferior from the viewpoint of translucency in comparison with the ITO film subjected to the heat treatment, the light extraction efficiency may be degraded, and the light-emitting light output of the light-emitting element may be reduced. In addition, since the amorphous IZO film has a high contact resistance against the p-type GaN layer, the driving voltage of the light-emitting element may increase. In addition, since it is amorphous, water-resistance properties or chemical resistance properties are degraded. A product yield in the manufacturing process after the IZO film is formed is degraded. Therefore, device reliability may be degraded.
Meanwhile, there has been proposed a light-emitting element obtained by providing the crystallized IZO film on the p-type semiconductor and used as the translucent electrode (e.g., refer to Patent Citation 4). In addition, Patent Citation 4 discloses that the sheet resistance decreases as the annealing temperature increases by annealing the amorphous IZO film at a temperature ranging from 300 to 600° C. under a nitrogen atmosphere not including oxygen (paragraph 0036, Patent Citation 4) and that it is recognized that the IZO film is crystallized because an X-ray peak representing In2O3 is mainly detected when the IZO film is annealed at a temperature greater than or equal to 600° C. (refer to paragraph 0038, Patent Citation 4). In addition, Patent Citation 4 discloses that a transmittance in the ultraviolet light range (having a wavelength of 350 to 420 nm) increases 20 to 30% when the IZO film is annealed at a temperature of 600° C. in comparison with the IZO film not subjected to the annealing process (refer to paragraph 0040, Patent Citation 4). In addition, in the light-emitting element having such an IZO film, a light-emission distribution on the light-emitting plane shows that the light is emitted from the entire surface of the positive electrode, the driving voltage Vf is 3.3 V, and the light-emitting light output Po is 15 mW (refer to paragraph 0047, Patent Citation 4).
A crystal structure of indium oxide (In2O3) can be classified into two different crystal systems, a cubical crystal system and a hexagonal crystal system. In the case of the cubical crystal system, the fact that a stable bixbyite crystal structure can be obtained under a pressure equal to or lower than a normal pressure is known in the art and disclosed in various documents. In addition, a liquid crystal display panel obtained by using a polycrystal indium tin oxide film having the aforementioned cubical bixbyite crystal structure has been proposed (e.g., refer to Patent Citation 5).
As described above, in order to improve the light-emitting property of the light-emitting element, it is necessary to further improve light extraction efficiency from the translucent electrode provided on the p-type semiconductor layer. However, in the configurations of the translucent electrode disclosed in Patent Citations 1 to 5, it was difficult to obtain sufficient translucency. It is desired to provide a compound semiconductor light-emitting element including a translucent electrode having higher translucency and excellent light-emitting property.
[Patent Citation 1] Japanese Unexamined Patent Application Publication No. 2005-123501
[Patent Citation 2] Japanese Unexamined Patent Application Publication No. 2000-196152
[Patent Citation 3] Japanese Unexamined Patent Application Publication No. 08-217578
[Patent Citation 4] Japanese Unexamined Patent Application Publication No. 2007-287845
[Patent Citation 5] Japanese Unexamined Patent Application Publication No. 2001-215523